Geologic units in Caledonia county, Vermont

Waits River Formation - carbonaceous phyllite and limestone member (Lower Devonian and Upper Silurian) at surface, covers 54 % of this area

Dark-gray to silvery-gray, lustrous, carbonaceous muscovite-biotite-quartz (±garnet) phyllite containing abundant beds of punky-brown-weathering, dark-bluish-gray micaceous quartz-rich limestone in beds ranging from 10 cm to 10 m thick. Part of the Connecticut Valley Trough.

Gile Mountain Formation - quartzite and metapelite member (Lower Devonian) at surface, covers 18 % of this area

Gray to light-gray, fine-grained micaceous quartzite a few centimeters to tens of centimeters thick, interbedded with dark-gray graphitic slate, phyllite, or schist. Part of the Connecticut Valley Trough.

Gile Mountain Formation - rhythmically graded member (Lower Devonian) at surface, covers 8 % of this area

Light- to medium-gray, fine-grained micaceous quartzite to dark-gray muscovite-quartz-biotite carbonaceous phyllite or schist in beds 10 to 25 cm thick; and dark-gray micaceous phyllite or schist containing beds of micaceous quartzite; locally thickly bedded. Detrital volcanic zircons yield a U-Pb age of 409±5 Ma, no. 51 (McWilliams and others, 2010). Part of the Connecticut Valley Trough.

Knox Mountain pluton (Devonian) at surface, covers 4 % of this area

Light-gray, medium-grained, quartz-rich biotite-muscovite granite. Has distinctly more potassium feldspar and is lighter gray than rocks of the East Barre plutons. Part of other plutons of the New Hampshire Plutonic Suite.

Albee Formation (Ordovician and Cambrian) at surface, covers 3 % of this area

Light-gray to greenish-gray, white-weathering, fine-grained feldspathic metasandstone and metasiltstone, and light-gray to greenish-gray to dark-gray phyllite. Lesser amounts of quartzite. Rare calc-silicate nodules. Generally sharply bedded, but graded beds as well as slump structures are locally obvious. Tourmaline is a sporadic accessory mineral. May be sulfidic (either pyrite or pyrrhotite) and rusty weathering. “Pinstriping” is common. U-Pb zircon SHRIMP age of 492.5±7.8 Ma from a porphyritic tonalite sill about 2 km east of West Bath, N.H. (D.W. Rankin, USGS, unpub. data, 2011). Stratified rocks of the Bronson Hill arch and Sawyer Mountain belt.

Devonian intrusive rocks (Devonian) at surface, covers 3 % of this area

Binary and biotite granite and granodiorite, undifferentiated. Includes small dikes labeled Dg. Part of the New Hampshire Plutonic Suite.

Gile Mountain Formation - thick-bedded micaceous feldspathic quartzite member (Lower Devonian) at surface, covers 2 % of this area

Brown to gray, noncarbonaceous quartz-mica schist and feldspathic quartzite in beds 50 cm to 5 m thick; gradational to Dgqs through interbedding of phyllite beds and decrease in thickness of quartzite beds. Part of the Connecticut Valley Trough.

Newark pluton (Devonian) at surface, covers 2 % of this area

Pinkish-gray to cream-colored, medium- to coarse-grained magnetite-microcline-perthite-biotite granite. Igneous rocks of the Northeast Kingdom batholith of Ayuso and Arth (1992). Part of the New Hampshire Plutonic Suite.

Moretown Formation - “pinstriped” granofels member (Lower Ordovician to Cambrian?) at surface, covers 2 % of this area

Light-gray to pale-green, whitish-gray-weathering, chlorite-biotite-plagioclase-quartz granofels and tectonically “pinstriped” granofels and feldspathic biotite quartzite. Part of the metasedimentary host rocks of the North River Igneous Suite. Rocks of the Early to Late Taconic Accreted Terrane of the Rowe-Hawley Zone. Eastern allochthonous sequence, oceanic and accretionary realm, ultramafic inclusions, volcanic-arc intrusives, and volcanic rocks.

Gile Mountain Formation - Meetinghouse Slate member (Lower Devonian) at surface, covers 0.9 % of this area

Dark-gray slate and phyllite containing sparse to moderately abundant beds of light-gray, fine-grained metasandstone and metasiltstone, 1 mm to 1 cm thick. Part of the Connecticut Valley Trough.

Northfield Formation (Lower Devonian and Upper Silurian) at surface, covers 0.9 % of this area

Dark-gray to silvery-gray, lustrous, fine-grained carbonaceous quartz-muscovite phyllite and silicic phyllite, and garnet-rich biotite-muscovite-quartz schist; contains millimeter- to centimeter-thick beds of gray quartzite and metasiltstone, and thicker beds of quartz-feldspar grit or quartzite near base. Transition zone into Waits River Formation west of the Guilford dome consists of as much as 5 percent beds of punky limestone. Part of the Connecticut Valley Trough.

Albee Formation - Scarritt member (Ordovician and Cambrian) at surface, covers 0.8 % of this area

Dark-gray slate interlayered with thin beds of light-gray, fine-grained micaceous and feldspathic metasandstone (typically ribby weathering). Abruptly graded beds <1 cm to 30 cm thick are locally common as is channeling and, in places, soft-sediment deformation. Commonly sulfidic and rusty weathering. Stratified rocks of the Bronson Hill arch and Sawyer Mountain belt.

Willoughby pluton - main zone (Devonian) at surface, covers 0.7 % of this area

Light-gray to pink, medium- to very coarse grained, massive, miarolitic garnet-muscovite peraluminous leucogranite. Pegmatitic main zone. Rb/Sr whole-rock age of 376±9 Ma (Ayuso and Arth, 1992). Igneous rocks of the Northeast Kingdom batholith of Ayuso and Arth (1992). Part of the New Hampshire Plutonic Suite.

Ammonoosuc Volcanics of Billings (1935) - metasiltstone and phyllite (Upper and Middle Ordovician) at surface, covers 0.5 % of this area

Medium-light-bluish-gray, medium-bluish-gray, medium-dark-gray, to medium-dark-greenish-gray metasiltstone and phyllite, and medium-gray feldspathic metawacke. Purple tinge common; coticule and magnetite locally abundant. Stratified rocks of the Bronson Hill arch and Sawyer Mountain belt.

Waits River Formation - Crow Hill Member of Hall (1959) (Lower Devonian and Upper Silurian) at surface, covers 0.5 % of this area

Gray quartzite and feldspathic quartzite, in part volcaniclastic and locally interbedded with amphibolite. Part of the Connecticut Valley Trough.

Comerford Intrusive Complex - metatholeiitic mafic dikes (Late Silurian) at surface, covers 0.4 % of this area

Abundant, foliated to weakly foliated, metatholeiitic mafic dikes; some sheeted. Shown as overprint. Part of the Lake Memphremagog Intrusive Suite (425±3 Ma to 418.5±2 Ma).

Victory pluton (Devonian) at surface, covers 0.3 % of this area

A western zone of medium- to fine-grained biotite-muscovite granite and minor hornblende-biotite tonalite and an eastern, more melanocratic zone of biotite-hornblende quartz monzonite like that in the Nulhegan pluton, as well as biotite granodiorite. Igneous rocks of the Northeast Kingdom batholith of Ayuso and Arth (1992). Part of the New Hampshire Plutonic Suite.

Waits River Formation - mafic member (Lower Devonian and Upper Silurian) at surface, covers 0.3 % of this area

Massive, coarse-grained hornblende-plagioclase gneiss and granofels; finely foliated hornblende-plagioclase amphibolite; actinolite-epidote-chlorite greenstone. Part of the Connecticut Valley Trough.

Ammonoosuc Volcanics of Billings (1935) - metarhyolite tuff, lapilli tuff, tuff breccia, and lava (Upper and Middle Ordovician) at surface, covers 0.3 % of this area

Greenish-gray, light-bluish-gray, or medium-bluish-gray metarhyolite tuff, lapilli tuff, tuff breccia, and lava. Generally porphyritic with 5 to 20 percent plagioclase and, in some places, quartz phenocrysts. Generally strongly foliated with waxy sheen on foliation surfaces. Stratified rocks of the Bronson Hill arch and Sawyer Mountain belt.

Comerford Intrusive Complex - pegmatitic metatonalite to metagabbro (Late Silurian) at surface, covers 0.2 % of this area

Nonfoliated to foliated pegmatitic metatonalite to metagabbro. Part of the Lake Memphremagog Intrusive Suite (425±3 Ma to 418.5±2 Ma).

Joslin Turn Tonalite (Early Silurian to Middle Ordovician) at surface, covers 0.2 % of this area

Greenish-gray to light-brownish-gray, medium-grained, weakly foliated metamorphosed tonalite. Primary minerals include quartz, plagioclase, biotite, magnetite, pyrite, and apatite; secondary minerals include chlorite, epidote, sericite, and calcite. Granophyric intergrowths of quartz and plagioclase. U-Pb zircon age of 469±1.5 Ma, no. 27 (Moench and Aleinikoff, 2003). Part of the Highlandcroft Plutonic Suite: Epizonal to mesozonal, foliated and metamorphosed (greenschist facies) plutons exposed northwest of the Ammonoosuc fault. Compositions range from granite to diorite to lesser amounts of gabbro. Part of the Bronson Hill arch intrusive rocks.

Willoughby pluton - western zone (Devonian) at surface, covers 0.1 % of this area

Light-gray to pink, medium- to very coarse grained, massive, miarolitic garnet-muscovite peraluminous leucogranite. Biotitic granodioritic western zone. Rb/Sr whole-rock age of 376±9 Ma (Ayuso and Arth, 1992). Igneous rocks of the Northeast Kingdom batholith of Ayuso and Arth (1992). Part of the New Hampshire Plutonic Suite.

Gile Mountain Formation - amphibolite member (Lower Devonian) at surface, covers < 0.1 % of this area

Hornblende amphibolite and hornblende-plagioclase-quartz granofels; interpreted as metabasaltic and volcaniclastic rocks. Part of the Connecticut Valley Trough.

Stiles Pond pluton (Devonian) at surface, covers < 0.1 % of this area

Gray, medium-grained, nonfoliated biotite-muscovite tonalite. Part of other plutons of the New Hampshire Plutonic Suite.

Ammonoosuc Volcanics of Billings (1935) - undivided (Upper and Middle Ordovician) at surface, covers < 0.1 % of this area

A heterogeneous unit of interlayered and interfingering metamorphosed volcanic, volcaniclastic, and sedimentary rocks. Compositions range from basalt to sodic rhyolite. Fragmental rocks dominate (tuff to tuff breccia), but include sparse mafic pillow lava and felsic lava. Sedimentary protoliths include dark-gray sulfidic shale, ironstone, siltstone, graywacke, volcanic conglomerate, and rare limestone. Stratified rocks of the Bronson Hill arch and Sawyer Mountain belt.

Biotite metadiorite (Devonian) at surface, covers < 0.1 % of this area

Biotite metadiorite in unit Dgq northeast of St. Johnsbury - Medium-light-bluish-gray, fine-grained, foliated and lineated. Part of other plutons of the New Hampshire Plutonic Suite.

Granite and trondhjemite dikes of Chester and Athens domes and west of the Connecticut Valley trough (Devonian) at surface, covers < 0.1 % of this area

Light-gray to white, garnet-muscovite-biotite granodiorite at Gassetts quarry, having a U-Pb zircon SHRIMP age of 392±6 Ma, no. 46 (Aleinikoff and others, 2011), from rims on Proterozoic-age cores (J.N. Aleinikoff, USGS, written commun., 2002); and whitish-gray muscovite-rich quartz monzonite and granodiorite, granite pegmatite, and aplite which occur as crosscutting nonfoliated dikes within the core of the domes and as folded and well-foliated dikes on the east and west flanks of the domes; also granite dikes and small stocks of very light gray to white, muscovite-rich, locally garnet-bearing, fine- to medium-grained biotite-muscovite quartz monzonite and granodiorite, locally orbicular, that intrude cover rocks east of the Green Mountain massif from Jamaica to Northfield, including the Liberty Hill locality and the granodiorite stock east of Plymouth. The latter contains inherited zircon with rims having an imprecise SHRIMP age of about 380 to 390 Ma (Aleinikoff and others, 2011); a similar dike south of Plymouth has a U-Pb zircon SHRIMP age of 365±5 Ma, no. 49 (Aleinikoff and others, 2011). These closely resemble undated white granodiorites of the Bethel area and dikes in the Chester and Athens domes; also resemble small dikes of granite, trondhjemite, and quartz monzonite largely west of the Connecti-cut Valley trough. Devonian granitic rocks of southern Vermont. Part of the New Hampshire Plutonic Suite.

Granite and Tonalite (Devonian) at surface, covers < 0.1 % of this area

Porphyritic, light-gray, hypabyssal biotite granite and tonalite in sill-like bodies - Contains phenocrysts of euhedral and embayed quartz, euhedral micro-cline with Carlsbad twinning (in granite), plagioclase, and euhedral biotite (±muscovite). Part of other plutons of the New Hampshire Plutonic Suite.

Derby pluton - granodiorite and tonalite (Devonian) at surface, covers < 0.1 % of this area

Light-gray, medium- to coarse-grained, massive, epidote-biotite-muscovite porphyritic granodiorite and minor tonalite. Rb/Sr whole-rock isochron age of 370±17 Ma (Ayuso and Arth, 1992). Igneous rocks of the Northeast Kingdom batholith of Ayuso and Arth (1992). Part of the New Hampshire Plutonic Suite.

Partridge Formation (Upper Ordovician) at surface, covers < 0.1 % of this area

Dark-gray to grayish-black, rusty-weathering sulfidic slate and phyllite interlayered with felsic volcanic rocks and tuffs, and amphibolite (Opa). Stratified rocks of the Bronson Hill arch and Sawyer Mountain belt.